The invention relates in general to optical communication systems and in particular to a receiver for an optical signal.
It is known that signal dispersion, for example polarization mode dispersion, is one of a number of limiting criterion in connection with optical communication systems, in particular in connection with fiber optical transmission links. Successive digital pulses are smeared along the optical link so that they are no longer distinguishable as well-defined pulses at a receiver. Instead, the pulses overlap what leads to a distorted optical signal with so-called inter-symbol interferences.
It is also known that a decision feedback equalizer can be used in the receiver to eliminate the inter-symbol interferences. For that purpose, the decision feedback equalizer may comprise a feedforward filter and a feedback filter comprising one or more taps, respectively, and a detector. The feedforward filter receives the distorted signal and the detector generates a corrected output signal. This output signal is input to the feedback filter and the difference between the outputs of the feedforward filter and the feedback filter is input to the detector. The two filters may be implemented as finite impulse response filters with adjustable coefficients. For adaptation purposes, these coefficients may be improved e.g. with a last mean square (LMS) algorithm. Reference is made to “John. G. Proakis: Channel Equalization, in J. D. Gibson: The Communication Handbook, CRC press/IEEE press, pages 339 to 363.
However, the known algorithms do not provide an optimum adaptation so that a resulting bit error rate is not minimized.
Furthermore, it is known to combine a decision feedback equalizer and an eye monitor in order to improve the elimination of inter-signal interferences. An example of this combination is disclosed in U.S. 2002/0060820 A1. There, in connection with FIG. 3, a digital decision feedback equalizer is described that consists of two decision elements being connected in parallel wherein the outputs of these decision elements are connected to a switch. The decision elements may be implemented as decision flip-flops. As it is also shown in FIG. 3, two eye monitors are present being connected in parallel and wherein each one comprises a further decision flip-flop.
The incoming distorted signal, therefore, is forwarded to four decision flip-flops. Due to the electrical input circuits of these four decision flip-flops, the distorted signal becomes even more corrupted and the bit error rate increases.